One thing's not clear from your post...Was the lack of pressure to get water to the top of the slide something that was occurring before you got the new gauge?
If you are getting a different reading on the new gauge vs. the old one, I would be more inclined to trust the new gauge.
When someone talks of a pump going bad, it is usually the motor as opposed to the pump itself, which we sometimes refer to as the "wet end". Usually what happens is the motor becomes louder and begins sounding like what some have described as marbles in a tin can. Usually that's a sign that the motor bearings are failing. As for the pump (wet-end) itself, it is possible that the impeller may have sustained some damage or has worn down enough to noticeably reduce flow, although this is a bit rare and would take an unusual amount of debris passing through the pump filter basket over a long period of time for this to happen.
It also could be that the current pump is fine, but is now insufficient to generate the pressure needed with the extra return, heater, and slide. If your current pump checks out fine, but it is determined that it does not have the power to generate the water flow you need with your current plumbing setup, I would replace the current pump/motor with a new packaged unit (pump & motor). You would definitely be better off with a 2-speed or variable-speed motor. While it's possible to replace just the motor on an existing pump, you would normally replace the motor with one of equivalent Service Factor HP as the motor and pump impeller must be matched in order to function properly. However, if more pumping power is needed when using the slide and all 3 returns, then simply replacing the current motor with an equivalent new one will not solve your problem. Therefore, a new pump/motor unit is what you would need. It may be worth the cost of a consulting fee to get a trusted expert out there to determine the size and power of the pump/motor that should be installed for your specific pool. You want a pump/motor that has the power you need and no more. The multi-speed motor will give you the flexibility to ramp it up when you need it and dial it down for basic water circulation when the slide is not in use.